Abstract
An aggressive, photocatalytically initiated, free-radical degradation mechanism promoted by specific components of modern sunscreen formulations is proposed for appearance of unsightly defects on prepainted steel sheets installed in roofing applications. The effect has been confirmed and reproduced in both laboratory and exterior exposure tests. X-ray diffraction (XRD) studies reveal the presence of a potent photocatalyst in several sunscreen formulations. Electron spin resonance (ESR) studies confirm the photocatalytic activity through monitoring production of hydroxyl radicals, HO , using the spin trapping technique. The model shows that surface coatings with an inherent roughness are highly susceptible to this effect. In practical terms, it is estimated that the weathering (in terms of deterioration of appearance properties) of the coating has been accelerated 100-fold by this photocatalytic degradation mechanism. Benchmark surface coatings for this application sector, based upon ‘fluoropolymer’ technologies, are also severely damaged in a short space of time.
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